Urea—The estimation of urea in cerebrospinal fluid may be carried out by either of the two methods described for blood urea, without modification.
As urea is readily diffusable, the concentration in the spinal fluid closely approximates to that in the blood, and nothing is to be gained by withdrawing fluid only to determine its urea content. For the same reason, if spinal fluid is already available, the estima- tion may be performed on this, rendering further withdrawal of blood from the patient unnecessary.

Glucose.—Glucose similarly may be estimated by any of the standard blood-sugar techniques. It may be mentioned, however, that in those conditions in which knowledge of the spinal fluid glucose level is most likely to be of diagnostic importance (i.e., meningitis, particularly of tuberculous origin), values below 60 mg. per 100 ml. are of common occurrence. Under these circumstances it is advisable to dilute the fluid only 1 in 5 or even 1 in 2-5 when preparing the filtrate. T h e subsequent calculation is adjusted accordingly.

The cerebrospinal fluid glucose varies according to the level in the blood, the permeability of the choroid plexus and the capillaries of the meninges, and the rapidity with which glycolysis takes place.
The usual normal range for cerebrospinal fluid glucose has not been accurately established, though it appears to be 20-30 mg. per 100 ml. lower than the blood-sugar. In adults normal values are from 50 to 75 mg. per 100 ml., whilst in children up to ten years of age a variation between 70 and 90 mg. per 100 ml. is more common. Reducing substances other than glucose account for about 4-0 mg. per 100 ml.
Clinically, spinal fluid glucose estimations find their most impor- tant application as an aid to the diagnosis of meningitis, particularly of tuberculous origin, in which condition glucose may virtually disappear from the subarachnoid space. In all types of meningitis the glucose is reduced, owing presumably to its utilization by the infecting organism and/or its hydrolysis by coexisting enzymes ; impairment of the permeability of the menigeal vascular endo- thelium may also play an important part in the cerebrospinal fluid glucose reduction which occurs in meningeal infections.

Frederick N. Bullock, 1954